Many variable speed drive mechanisms of various designs are described in the literature or are commercially available. These mechanisms find application in fields as diverse as computers, machine tools, recreational vehicles, construction equipment, trucks and automobiles. They all share the basic function of converting the rotational speed and torque of an input shaft to a selected variable speed and torque at an output shaft.
Small recreational vehicles and personal transportation vehicles are ideal applications for an infinitely variable speed drive mechanism because of the improved economy, longer service life and low exhaust emissions that can be obtained by operating the vehicle's prime mover, such as an internal combustion engine, in the range of its optimum operating point and varying the speed of the vehicle by adjusting the transmission over a continuous range instead of changing the vehicle engine speed. Use of an efficient infinitely variable transmission in a large number of vehicles would save an inestimable amount of fuel and reduce the world-wide exhaust emissions more than any other know conservation and air purity stratagem now considered feasible.
Although many infinitely variable transmissions and continuously variable transmissions have been proposed and designed for vehicular applications, none has proven entirely satisfactory. Traction devices have been unable to demonstrate acceptable life and the transient torque conditions occurring in a normal vehicle driving cycle. Rubber belt variator devices, such as the centrifugal clutch commonly used in snowmobiles, do not offer adequate durability and efficiency, even in light vehicles with engine power on the order of only ten to fifteen horsepower. The most common application for this type of transmission is the snowmobile, where component life is not expected to exceed 100 hours.
Hydrostatic transmissions have existed for years and have been developed to a high degree of sophistication. These devices are in use in some military, agriculture and construction equipment, mining and other off-the-road vehicles, and in small garden tractors. A conventional hydrostatic transmission has two principal elements: a hydraulic pump driven by the prime mover, and a hydraulic motor powered by hydraulic fluid pressurized by the pump for driving the load. Either or both of these elements may be variable displacement to achieve the variable gear ratio of the transmission. Regardless of the configuration selected, the overall system efficiency can be no better than the product of the efficiencies of the individual elements. For example, if both the pump and motor are 95% efficient, the hydrostatic unit cannot achieve efficiency greater than (0.95.times.0.95)=90% and in practice it is usually significantly less than this because of flow losses in the hydraulic lines coupling the two elements. This efficiency is inferior to that offered by conventional automatic transmissions which can operate at steady state efficiency levels on the order of 97%-98% with torque converter lock-up, but the advantages of an infinitely variable transmission and the absence of a clutch outweigh the disadvantage of low efficiency in the applications in which conventional hydrostatic transmissions have been used successfully.
In addition to their low operating efficiencies, there are other disadvantages that have militated against the wide use of conventional hydrostatic transmissions. They are usually bulky, heavy and expensive. In addition, conventional hydrostatic transmissions are noisy, especially at the higher gear ratios where most over-the-road driving is done because the flow rate of the hydraulic fluid is greatest at the high gear ratios in these hydrostatic transmissions.
The integrated hydrostatic transmission, in which the motor and pump are combined in one unit to minimize fluid flow losses, is a step in the right direction. However, none of the prior art integrated hydrostatic transmissions overcome the condition which degrades their efficiency and contributes to their noisiness, namely, that the peak power rating of the transmission is attained at maximum pressure and flow. As a consequence, hydraulic losses associated with pressure, such as leakage and hysterisis losses during fluid compression and expansion will be greatest at maximum power throughput. Also, viscous flow losses which are proportional to fluid velocities are greatest at peak power/speed when the flow and pressure are at their highest levels.
The lack of enduring commercial uses of hydrostatic transmissions in production for vehicle or other uses that require a high power-to-weight ratio is believed to be due to four main reasons: 1) high cost, 2) high noise levels at normal operating conditions, (3) poor efficiency, and (4) lack of any significant weight and size advantage. However, modern production techniques have been developed that would make it possible to produce a hydrostatic transmission designed specifically for such applications at a cost approximately comparable to that of a prior art adjustable ratio variable transmission. The second and third factors, namely, noise and efficiency, have been the key factors discouraging adoption of a hydrostatic transmission by the recreational and personal vehicle industries. The size and weight factors could be significant if there were competing designs that satisfied the first three factors.
One effort to overcome some of the disadvantages of the conventional hydrostatic transmission is the power branching transmission. An early example of such a transmission is shown in U.S. Pat. No. 3,175,363 to Hans Molly. The power branching transmission was intended to reduce the fluid flow losses associated with the hydrostatic transmission, particularly as the transmission ratio moves toward unity, by transmitting a portion of the input power mechanically to the output shaft. Since the proportion of mechanically transmitted power increases to 100% at a 1:1 transmission ratio, the hydraulic losses are potentially much less in a power branching transmission.
Unfortunately, attempts to commercialize the power branching hydrostatic transmission have been unsuccessful, probably because the complexity of the system would compromise performance and increase cost to a noncompetitive level versus the conventional transmission. Also, the prior art power branching transmissions have not been able to achieve a dynamic balance of the rotating elements which would be a serious shortcoming since substantial vibration levels at operating speed would not be acceptable. In addition, prior art power branching transmissions have not been readily scaleable to make different sizes of a single design usable for different power ranges. Scalability could be an important feature in smaller applications such as snowmobiles and motorcycles where the ability to match the size, weight and cost of the transmission precisely with the power, torque and speed requirements could become competitively important.
If the power available in operation of a vehicle during braking and periods of low power requirements could be stored and made available for use during periods of auxiliary or high power requirements such as hydraulic power take-off, engine starting, and vehicle acceleration, the engine sizing for any given vehicle could be reduced substantially, since engines are normally sized for the maximum anticipated power requirements. The storage of hydraulic energy in an accumulator is a well known and understood technology and should encounter no resistance to use in motor vehicle applications as some new technologies have in the past, and the use of a moderately sized accumulator will add little weight and cost, certainly less than is saved by the use of a small light weight compact vane-type continuously variable transmission that makes possible the elimination of a clutch and a starter motor and makes possible the substantial downsizing of the engine because of the availability of the added hydraulic power source.
Thus, the transmission art has long needed an improved infinitely variable hydrostatic transmission that provides the advantages of the integrated hydrostatic transmission while markedly improving efficiency by reducing the hydraulic fluid losses associated with conventional hydrostatic transmissions, reducing the size, weight, cost, emissions and noise levels of operation, improving the performance near the neutral point, offering scalability of some basic machine designs, and reducing the manufacturing and maintenance costs.